1. Field of the Invention
The present invention relates to light scanning systems for reading data. In preferred embodiments the invention is useful in compact moving spot scanning systems for reading bar code symbols.
2. Description of Related Art
Various optical scanning systems have been developed for reading bar code symbols. In many such systems a laser spot is swept across the bar code. The bar code symbols reflect or absorb the incident light. A photo detector receives reflected light and produces a signal from which the data is obtained. Normally, in such systems the laser spot scans the bar code symbols in such a way that it crosses all of the bars and spaces which make up the code. These various optical scanning systems require apparatus for sweeping the spot across the bar code symbol.
For this purpose, it is known to use a stationary source of light, a scanning mirror and a driver to repetitively oscillate the mirror, so that an incident light beam from the source is repetitively swept through an angular distance at a rate, for example, of tens of oscillations per second. Such a system is shown, for example, in U.S. Pat. No. 4,816,660 to Swartz et al. An electric motor may be used to drive the mirror. Alternatively, a swept beam may be produced by a semi conductor laser whose output beam is directed by a piezoelectric driver. Such a system is shown in U.S. Pat. No. 5,625,483 to Swartz et al. Magnetic fields may also be used to drive small mirror elements.
In motor driven mirror scanning systems, conventional motor controls are used to insure the appropriate scanning. In these systems, the components, such as photo diodes, mirrors, motor parts and control circuits are separately manufactured and then assembled. The components must be accurately aligned for proper operation. These production steps are costly. Use of a micro machined permits partial or total integration of the scanner into a miniature package with components formed on one or a few substrates. However, in a system based on the use of a driven, micro machined scanner element, there is the need to insure the proper movement of the scanner in a way which is compatible with the micro machined parts in terms of size and structure, and in terms of the manufacturing steps used to make the micro machined parts.
Micro machined scan mirrors have been described in prior art literature. However, there are unique challenges in designing a scan mirror for bar code scanning. In particular, the mirror is advantageously several millimeters on each side to accommodate the scan beam. Handheld bar code scanners operate at very low frequencies, scan large angles, and need to survive large shock loads. The mirrors may be operated at resonance in order to reduce power consumption. Low resonant frequency dictates a large mass on relatively weak suspension, which makes it difficult to provide the required shock resistance. Another important requirement is low cost.
A micro machined mirror for scanning a light beam is disclosed in U.S. patent application Ser. No. 08/483,163 filed Jan. 7, 1995 to Swartz et al. entitled AN INTEGRATED SCANNER ON A COMMON SUBSTRATE, which is hereby incorporated by reference. In this system, the mirror may be driven by various techniques including electrostatic actuation, electromagnetic actuation, and heat actuation. However, such mirrors are difficult to operate due to the high Q of the mechanical system and the drive requirements for such a system.